A new energy frontier for heavy ions

Although they lack the fundamental significance of the constants of Nature, powers of 10 have symbolic importance in human culture. Attaining the age of 100 years is rare enough to be celebrated in many ways around the world. We regularly mark the 100th anniversaries of historical events. A fortunate few can celebrate becoming a millionaire or billionaire.

So also with particle accelerators. Two of the great machines of the past were named after the symbolic energy barrier that they broke. The Bevatron (for "billions of electron volts synchrotron"), at Berkeley in 1954, was the first to break the barrier of a billion electron volts (now known as a giga-electron volt or GeV) in the centre-of-mass, by enough to create the first anti-protons in the laboratory (other early synchrotrons, in Birmingham and Brookhaven, accelerated beams to GeV-scale energies around the same time).

Three decades later, in 1987, the Tevatron at Fermilab breached the barrier of 1 tera-electron volt or TeV, a trillion electron-volts or 1000 GeV, in the centre-of-mass. This was thanks to the use of superconducting magnets, colliding beams rather than fixed targets and, of course, the beautiful mathematics of the strong-focussing principle. Together, these made large rings of moderate-sized high-field magnets affordable*. The Tevatron beam energy itself was almost 1 TeV, yielding almost 2 TeV in the collisions of opposing beams.

Earlier this year, the LHC broke its own previous record by colliding protons at 13 TeV in the centre-of-mass, thanks to even higher-field magnets and an even larger ring. Its previous 7-8 TeV, before the improvements of the first long shutdown, was enough to create the Higgs boson.

This week, just under three decades since the Tevatron reached 1 TeV, the LHC resumed its programme of colliding so-called "heavy ions". More precisely, these are the nuclei of lead atoms. Since these nuclei contain the electric charges of 82 protons, the machine can accelerate them to 82 times the energy**, reaching 1045 TeV in the collisions, breaking the symbolic barrier of a quadrillion electron-volts, or 1 PeV (peta-electron volt ).

However, the lead isotope accelerated in the LHC also contains 126 neutrons which have no electric charge for the LHC's accelerating fields to work on. So the energy is shared among 208 nucleons, each of which receives 82/208 or 39.4% of the energy that the LHC imparts to single protons. In the nuclear-physics literature, it is customary to quote the average centre-of-mass energy of pairs of colliding nucleons, which is now 5.02 TeV. Still, the concentration of so much energy into the tiny nuclear volume is enough to establish truly colossal densities and temperatures about a quarter of a million times those at the core of the sun. Heavy-ion collisions recreate the quark-gluon plasma, the extreme state of matter that is believed to have filled the universe when it was only microseconds old. Besides searching for new fundamental particles, the LHC experiments have substantial physics programmes that study the collective behaviour of quarks and gluons when they form this state.

From the perspective of the early 1950s, the energies attained by the Tevatron and LHC would have seemed like science-fiction. But, thanks to breakthroughs in accelerator science and technology in subsequent decades, they are now real. Who can anticipate the future progress that may one day give us single nucleons colliding with 1 PeV ?

In the meantime, while it seems unlikely that the LHC will be renamed the "Pevatron", we can celebrate this week's breaking of a new symbolic energy barrier. It may be a while before the next such occasion.

*The Bevatron had a single 10,000 tonne magnet. Each of the LHC's 1232 bending magnets weighs 35 tonnes. So, for about 5 times the weight of the Bevatron magnet, the LHC provides about 10,000 times the (proton) collision energy. This is another measure of the progress of accelerator science and technology in the meantime.

**If you check the arithmetic, you will see that the lead beam energy is actually very slightly less than 82 times the proton energy. This year, the LHC experiments requested the same average energy per pair of colliding nucleons as in the 2013 run where 4 TeV protons were collided with 328 TeV lead nuclei. This was arranged by lowering the LHC's magnetic field from the value corresponding to 6.5 TeV protons to that of 6.37 TeV protons. That was still just enough to exceed 1 PeV in total collision energy!